code: def test_histogram_mapping(marginal_layer_spec, expected_mapping):
      input = marginal_layer_spec.as_dict()
      layer_mapping = input.get("mapping", {})
  
      assert layer_mapping == expected_mapping
  
file_path: lets-plot/python-package/test/plot/test_marginal_layer.py
imports: import pytest
  import lets_plot
label_desc: Use image node_id 8 for calling a global function with example usage: test_histogram_mapping(marginal_layer_spec, expected_mapping) without return types